For many years, the preferred process for color printing on paper has been to process the page in multiple exposures, or separations. Each separation exposes selected areas of the paper to an ink of a different color--the usual process ink colors are cyan, magenta, yellow and key (CMYK). Additional color inks are often used as spot colors in additional separation.
High-quality printing, such as that required by the publishing industry, poses many difficult problems in controlling the separations. For example, there is often a problem with positional registration of the various separations. As a result, the several inks are never perfectly aligned with one another, and a small gap between regions of different colors occurs, which is often quite visible to the eye as a white line.
One common solution to the registration problem is to perform a technique known as trapping. As the separations are built, regions of a particular color are expanded or "spread" beyond their nominal boundaries, or contracted or "choked" within such boundaries. Various photographic trapping techniques are well known. Although these are sometimes tedious to apply, they are justified in applications such as magazine and newspaper publishing, where the time and labor is available to create individual traps for each printed page.
With the current widespread use of computers in publishing, various systems now perform choking and spreading electronically. The usual approach is to first render the page in pixels at the desired output resolution and to store the rendition in a memory referred to as a "frame buffer". Typically, a frame buffer is assigned to each of the four process separations. If spot colors are used, additional frame buffers are necessary for each of these as well. Each frame buffer is then choked and spread on a pixel-by-pixel basis, and then used to control the printing of its respective color. While this approach requires a frame buffer for each of the output inks, the cost of such hardware is justified in newspaper and magazine publishing.
In recent years, a new class of low-cost applications in the publishing industry have emerged, namely "desktop" publishing. A desktop publishing system user typically uses a standard personal computer to specify his output page as a computer file in a page description language (PDL) such as "Postscript". (Postscript is a trademark of Adobe Systems, Incorporated of Mountain View, Calif.) The Postscript PDL file is then typically sent to a PDL-compatible automatic page setter, which then interprets the file and renders the several color separations.
The typical PDL user usually does not know or care about registration of separations--all they understand is that the color page as originally displayed on the computer screen often does not look the same as the page returned by the printing press. The desktop publishing user can manually examine the page on a computer screen, predict where the registration errors are likely to occur, and then specify additional vectors to cover the errors. However, this is tedious and undesirable to perform manually, and difficult to automate, especially for pages having multiple intersecting colored objects.
Thus, what is needed is a way for a desktop publishing user to automatically produce a properly trapped page from a PDL file. This would provide predictable results from a desktop publishing system without the need to provide special instructions to the page setter to incur the cost of using expensive trapping hardware.